I actually studied this in grad school - the slots are set up to give you back 60 - 80% of your money, depending on how sleezy the joint is. They don’t want you to lose big, they want you to come back next payday.
The occasional big payout, for them, is just an advertising expense.
Brings in more folks willing to lose.
Your studies were wrong. I speak with 7 years design experience in slot games and have personally worked on Cleopatra games.